1. Field of Invention
This invention relates generally to magneto-optic information storage systems and, more particularly, to improvements in the apparatus for interacting with the magneto-optic information storage system to determine the magnetic orientation or state of a selected region of the magneto-optic information storage system.
2. Description of the Related Art
The technique for retrieving information stored in the form of oriented magnetic regions from a magneto-optic storage medium using the Kerr (or the Faraday) effect is known in the related art and can be summarized as follows. The magneto-optic storage medium, having information previously recorded thereon, is scan irradiated with linearly polarized radiation beam (typically from a laser diode). Interaction with the storage medium results in a small clockwise or counterclockwise rotation, typically of the order of 1.degree. or less in the plane of polarization of the reflected or transmitted radiation. The direction of the rotation is determined by the vertical orientation (either up or down) of the irradiated magnetic domains which are indicative of the recorded information. If the impinging linearly polarized radiation is considered to be a combination of two in-phase components, a left-handed circularly polarized (LCP) component and a right-handed circularly polarized (RCP) component, then the resulting Kerr rotation of the linearly polarized radiation beam can be understood as being the result of a media-induced difference in phase retardation between the LCP and the RCP components.
As indicated above, the amount of Kerr rotation produced by a magneto-optic storage medium is relatively small and various schemes have been proposed to enhance the detectability of the direction of rotation. In U.S. patent application Ser. No. 07/319,031 filed on Mar. 6, 1989, entitled MAGNETO-OPTIC READOUT METHOD AND APPARATUS USING POLARIZATION SWITCHING OF READOUT BEAM, invented by C. N. Kurtz and J. J. Miceli, Jr, and assigned to the assignee of the present application, a linearly polarized beam of radiation, such as is emitted by a laser diode, is converted to circularly polarized radiation, either RCP or LCP, prior to application to a magneto-optic storage media. Upon being reflected from the medium, the radiation beam is reconverted to a linearly polarized radiation beam parallel to the original linear polarization of the beam. Depending on the orientation of the domains of the medium being irradiated, the circularly polarized radiation will be reflected (or absorbed) as a function of the orientation of the film due to the phenomenon known as magnetic-circular dichroism. When the first mode of the laser and a second (orthogonal) mode of the laser (i.e., the TE and TM modes for a laser diode) are converted to circularly polarized radiation beams, then, when the medium is considered as part of the cavity in which the modes of the laser operate, the orientation of the domains of the storage magneto-optic media will result in a differential decrease in the amplitude of radiation for each mode depending on the laser mode. If the parameters of the laser cavity are adjusted so that the cavity is equally likely to operate in either orthogonal radiation mode, then the orientation of the domains irradiated by the two modes will control which mode is selected for operation.
The apparatus for providing a mode switching type of magnetic orientation for a magneto-optic material has been described by Kurtz and Miceli (cited above) and is shown in FIG. 1. A radiation beam B from emitted from the front face (FF) of a laser L having two (linear and orthogonal) modes of operation is collimated by lens CL and passed through a loss control element LC. Above and below the apparatus shown in FIG. 1 are the states of polarization of the radiation beam during passage through the elements of the read apparatus. The loss control element LC functions to selectively increase the radiation losses in the normally dominant mode of laser operation, i.e., the TE mode for a laser diode, in the extended laser cavity. The losses for the dominant mode of laser operation are adjusted in such an manner that either mode of operation is equally likely. The radiation from the loss control element LC is passed through a polarization converter PC wherein the linearly polarized radiation components are converted into circularly polarized radiation components. In FIG. 1, left-handed circular polarization, LPC, is shown as the result of interaction with the polarization converter. The circularly polarized radiation is focussed on the recording layer RL of the magneto-optic storage medium M by focussing lens FL. The radiation beam is reflected from the recording layer RL and becomes right circularly polarized, RCP. The reflected radiation beam is recollimated by the lens FL and applied to the polarization converter PC. The polarization converter PC restores the linear polarization of the radiation beam to be parallel with the original radiation beam polarization. The restored linear polarized radiation is focussed on the laser L. The extended cavity of the laser can include a reflector R, a loss control element LC, and a rear collimating lens RCL for a radiation beam extending from the rear surface RF of the laser L. It is important that the linear polarization be restored in order that the losses arising from the recording layer are coupled to the mode of laser operation and not be dependent upon the other loss components of the laser cavity. In this manner, the mode of operation of the laser can be determined by the orientation of the irradiated domains, or, stated in another manner, the mode of operation of the laser can be used to determine the orientation of the magnetic domains currently being irradiated by the radiation beam.
One technique for adjusting the losses in the laser cavity wherein the orthogonal modes of operation are in a state of indifferent equilibrium in an absence of the oriented magneto-optic storage material is to replace the magneto-optic film (RL) with a reflector having a reflectance approximately equal to the average reflectance of the recording layer. Then the loss control element (LC) can be adjusted until the orthogonal modes of operation of the laser, including the extended cavity, are in a state of indifferent equilibrium. The loss control element must typically attenuate the two orthogonal modes of operation to provide for the equal possibility of laser operation in either mode, but not attenuate either component to the extent that the amplification of the laser element can not over come the attenuation. The loss control element can be a rotatable polarizer RP placed between the rear laser surface RF and reflector R as shown by dotted lines in FIG. 1. The angle of the polarizer will determine the losses added to the orthogonal radiation modes of the cavity. The loss control element LC can be a (coated) glass plate which is tilted to provide a differential reflection (loss) for the two orthogonal radiation modes. Optical filters can be used to adjust the two losses of the two orthogonal modes of the laser or a compensating coating applied to the front surface of the laser can be used to equalize the probability of laser operation in each of two orthogonal modes.
However, the adjustment of the two modes of laser operation to provide for the equalization of the probability of laser operation is complicated by the fact that the loss control element is in the path of each mode of laser operation. This position of the loss control element results in difficulty in the equalization of the two modes of laser operation and can introduce undesired losses in the mode for which attenuation is not required,
A need has been felt for apparatus and an associated method by which the attenuation of the extended cavity of a laser diode can be controlled to implement polarization mode switching in magneto-optic readout apparatus.